Yesterday I watched several hours of Humankind streams and got the avatars of two personas, but no beta key, even, though, according to Twitch I "got it" (Redeemed "Humankind - Closed Beta "). There is no key on my rewards page, no key sent to my e-mail, no game on my Steam account, nothing. Yes, of course the accounts are linked.

Why is this still promoted on Twitch, then? I just contacted Twitch support to let them know of this issue.

Try refreshing the G2G link to your twitch account by hovering over the Humankind tab to the left, and going to "Twitch Drops." Then click on "Account Linked" and hit the refresh button next to "Twitch Account." Afterwards, it should show up on your rewards page.

Another thing: it appears you have posted in the wrong forum. 

Moved to the correct forum